The present invention relates generally to radiocommunication systems, and more particularly to the use of Code Division Multiple Access (CDMA) communication techniques in a radio communication system. The invention relates even more particularly to methods and apparatuses for determining an existing level of service capability in a CDMA system.
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is rapidly outstripping system capacity. If this trend continues, the effects of this industry""s growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as to maintain high quality service and avoid rising prices.
Throughout the world, one important step in the advancement of radio communication systems is the change from analog to digital transmission. Equally significant is the choice of an effective digital transmission scheme for implementing next generation technology. Furthermore, it is widely believed that the first generation of Personal Communication Networks (PCNs), employing low cost, pocket-sized, cordless telephones that can be carried comfortably and used to make or receive calls in the home, office, street, car, and the like, will be provided by, for example, cellular carriers using the next generation digital cellular system infrastructure. An important feature desired in these new systems is increased traffic capacity, and efficient use of this capacity.
Currently, channel access is very often achieved using Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) methods. In FDMA, a communication channel is a single radio frequency band into which a signal""s transmission power is concentrated. Signals that can interfere with a communication channel include those transmitted on adjacent channels (adjacent channel interference) and those transmitted on the same channel in other cells (co-channel interference). Interference with adjacent channels is limited by the use of band pass filters which only pass signal energy within the specified frequency band. Co-channel interference is reduced to tolerable levels by restricting channel reuse such that a minimum separation distance is required to exist between cells in which the same frequency channel is used. Thus, with each channel being assigned a different frequency, system capacity is limited by the available frequencies as well as by limitations imposed by channel reuse.
In TDMA systems, a channel consists of, for example, a time slot in a periodic train of time intervals over the same frequency. Each period of time slots is called a frame. A given signal""s energy is confined to one of these time slots on a given frequency. Adjacent channel interference is limited by the use of a time gate or other synchronization element that only passes signal energy received at the proper time. Thus, with each channel being assigned a different time slot, system capacity is limited by the number of available time slots as well as by limitations imposed by channel reuse as described above with respect to FDMA.
With FDMA and TDMA systems (as well as hybrid FDMA/TDMA systems), one goal of system designers is to ensure that two potentially interfering signals do not occupy the same frequency at the same time. In contrast, Code Division Multiple Access (CDMA) is a channel access technique that allows signals to overlap in both time and frequency. CDMA is a type of spread spectrum communication technique, which has been around since the days of World War II. Early applications were predominantly military oriented. However, today there has been an increasing interest in using spread spectrum systems in commercial applications because spread spectrum communications provide robustness against interference, which allows for multiple signals to occupy the same bandwidth at the same time. Examples of such commercial applications include digital cellular radio, land mobile radio, and indoor and outdoor personal communication networks.
In a CDMA system, each signal is transmitted using any of a number of spread spectrum techniques. In some variations of CDMA, the informational data stream to be transmitted is impressed upon a much higher rate data stream known as a signature sequence. Typically, the signature sequence data are binary, thereby providing a bit stream. One way to generate this signature sequence is with a pseudo-noise (PN) process that appears random, but can be replicated by an authorized receiver. The informational data stream and the high bit rate signature sequence stream are combined by multiplying the two bit streams together, assuming the binary values of the two bit streams are represented by +1 or xe2x88x921. This combination of the higher bit rate signal with the lower bit rate data stream is called spreading the informational data stream signal. Each informational data stream or channel is allocated a unique signature sequence.
A plurality of spread information signals modulate a radio frequency carrier, for example by binary phase shift keying (BPSK), and are jointly received as a composite signal at the receiver. Each of the spread signals overlaps all of the other spread signals, as well as noise-related signals, in both frequency and time. If the receiver is authorized, then the composite signal is correlated with one of the unique signature sequences, and the corresponding information signal can be isolated and despread. If quadrature phase shift keying (QPSK) modulation is used, then the signature sequence may consist of complex numbers (having real and imaginary parts), where the real and imaginary parts are used to modulate respective ones of two carriers at the same frequency, but ninety degrees out of phase with respect to one another.
Traditionally, a signature sequence is used to represent one bit of information. Receiving the transmitted sequence or its complement indicates whether the information bit is a +1 or xe2x88x921, sometimes denoted xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d. The signature sequence usually comprises N bits, and each bit of the signature sequence is called a xe2x80x9cchipxe2x80x9d. The entire N-chip sequence, or its complement, is referred to as a transmitted symbol. The conventional receiver, such as a RAKE receiver, correlates the received signal with the complex conjugate of the known signature sequence to produce a correlation value. Only the real part of the correlation value is computed. When a large positive correlation results, a xe2x80x9c0xe2x80x9d is detected; when a large negative correlation results, a xe2x80x9c1xe2x80x9d is detected.
The xe2x80x9cinformation bitsxe2x80x9d referred to above can also be coded bits, where the code used is a block or convolutional code. Also, the signature sequence can be much longer than a single transmitted symbol, in which case a sub-sequence of the signature sequence is used to spread the information bit. In many radio communication systems, the received signal includes two components: an in-phase (I) component and a quadrature (Q) component. This occurs because the transmitted signal has two components (e.g., QPSK), and/or the intervening channel or lack of coherent carrier reference causes the transmitted signal to be divided into I and Q components. In a typical receiver using digital signal processing, the received I and Q component signals are sampled and stored at least every Tc seconds, where Tc is the duration of a chip.
CDMA techniques exist in a number of variants. Direct-sequence CDMA (DS-CDMA) operates as described above. Consequently, in DS-CDMA, the broadband frequency channel can be reused in every adjacent cell. Frequency-hopping techniques can also be employed to yield CDMA systems (FH-CDMA). Here, the hopping pattern can be formed as a code sequence. That is, a bit is sent on a pseudo-random pattern of frequency channels, and each subsequent bit is sent on a different pseudo-random pattern of frequency channels. The multiple frequency channels form a code for one bit. The code may be sent out simultaneously or sequentially. A larger bandwidth is required if each bit is to be sent out on the different frequency channels simultaneously, compared to the conventional FH-CDMA strategy of sending bits over frequency channels sequentially.
The traffic capacity of a CDMA system can be increased by managing power intelligently within the cell. Even with intelligent power management, however, there is an upper limit on the total amount of ongoing traffic that can take place in any one cell. This limit is related to the xe2x80x9cprocessing gainxe2x80x9d, which represents the ratio of the bandwidth per channel to the information transmission rate. In CDMA, different processing gains are achieved, depending on the symbol rate and the coding scheme employed. The processing gain for a particular service also affects what coverage can be achieved for that service.
The information transmission rate is often not fixed within a system, but instead may vary in dependence on the type of service being provided. For example, one type of service, such as voice transmission, may require the capacity to transmit at one information rate, while other services (such as movies, single picture, hi-fi music, fax and data) may each require the capacity to transmit information at very different rates. It is essential for a cellular subscriber to know if there is coverage at his present location for a particular service that he may wish to use.
Presently, second generation cellular systems provide the user with a single signal strength detection and indication. However, this is not sufficient to indicate the coverage and availability of different services.
One way to deal with this problem is to compensate for the varying processing gain by requiring the mobile station (MS) (and presumably also the network) to utilize an output power level that is inversely proportional to the processing gain, that is, higher output power would be used for services that have a lower processing gain, and vice versa. This solution is problematic because it means that bearer services having a high processing gain are prevented from using the full output power capability of the MS. As a consequence, the coverage for these services is not as good as it could be.
It is therefore an object of the present invention to provide methods and apparatuses that are capable of determining whether a service is available for use by a mobile station in a radiocommunication system.
It is another object of the invention to provide methods and apparatuses that are capable of determining an expected quality level of a service that is available for use by a mobile station in a radiocommunication system.
These and other objects are achieved in methods and apparatuses for determining whether a service is available for use by a mobile station in a radiocommunication system. In accordance with one aspect of the invention, this is achieved by determining a quality value that represents the mobile station""s transmission power capability that is above an amount of transmission power that will be consumed by known losses for the service. Whether the service is available for use by the mobile station is then determined on the basis of a comparison between the quality value and a predetermined number.
In another aspect of the invention, the technique for determining the quality value comprises subtracting the amount of transmission power that will be consumed by known losses for the service from a maximum available power that the mobile station can deliver.
In other aspects of the invention, the known losses may include any combination of the following: the power loss on a channel between the mobile station and a base station; the amount of power that is needed to overcome interference; and the amount of power needed to accommodate the service.
In another aspect of the invention, after the determination is made, an indication of whether the service is available may be displayed on the mobile station.
In yet another aspect of the invention, after the determination is made, the quality value may be used as an indication of an expected quality of the service. This indication may be displayed on the mobile station.
In still another aspect of the invention, the service may be activated in response to a determination that the service is available for use by the mobile station.
In yet another aspect of the invention, an activation of the service may be aborted in response to a determination that the service is not available for use by the mobile station.
In still another aspect of the invention, the quality value may be used to determine whether to change operation of the mobile station from the service to a different service, or from a different service to the service.